Electric power supply system

ABSTRACT

An electric power supply system can determine a sharing ratio of an electric power so as to increase and decrease an output electric power supplied by an electric power generator in accordance with an output electric power value required for the electric power supply system, in a fuel cell following region where a frequency of a magnitude of the electric power is equal to or higher than a predetermined value in a frequency distribution of a magnitude of the electric power, and can determine the sharing ratio of the electric power so as to increase an output electric power supplied by an electricity storage device, in an assist region where the frequency is lower than a predetermined value in the frequency distribution, and can prevent an excess of discharging from an electricity storage device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/128,461, filed May 28, 2008, which claims the foreign prioritybenefit under Title 35, United States Code, §119 (a)-(d), of JapanesePatent Application No. 2007-140732, filed on May 28, 2007 in the JapanPatent Office, the disclosure of which is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an electric power supply system havinga power generator.

For an electric power supply, JP2006-196221A discloses a fuel cellsystem. The fuel cell system computes an electric power generatingproperty of a fuel cell on the basis of an output voltage and an outputcurrent of the fuel cell. The fuel cell system computes an outputelectric power of the fuel cell system, which can be supplied to anelectrical load and computes the efficiency of the fuel cell system onthe basis of the output electric power and an energy of combustion for afuel gas of the fuel cell. Further, the fuel cell system computes anaccumulative energy efficiency in charging a secondary battery(chargeable device). The fuel cell system computes a total systemefficiency based on the output electric power, the energy efficiency ofthe secondary battery, and an output electric power required for thefuel cell.

The fuel cell system determines a distribution of the output electricpower between the fuel cell and the secondary battery (chargeabledevice) in order to maximize the total system efficiency. In otherwords, the fuel cell system determines the distribution of the outputelectric power between the fuel cell and the secondary battery(chargeable device) so as to minimize a total loss, which is a sum of afuel cell loss and a secondary battery loss.

However, the fuel cell system disclosed in JP2006-196221A has an problemin that the secondary battery (electricity storage device) excessivelydischarges electricity because the output electric power for the fuelcell and the secondary battery is distributed so as to minimize the fuelcell loss and the secondary battery loss. As the secondary battery lossis smaller than the fuel cell loss, a large amount of the outputelectric power is distributed to the secondary battery.

BRIEF SUMMARY OF THE INVENTION

An aspect of the present invention provides an electric power supplysystem which can prevent an excess of electricity discharged by anelectricity storage device.

Preferably, an electric power supply system comprising: an electricpower generator for generating an electric power; an electricity storagedevice for receiving and charging the electric power from the electricpower generator, and supplying the electric power to an electrical load;an electric power distributing device for distributing the electricpower to the electrical load, from the electric power generator and theelectricity storage device at a sharing ratio therebetween; and acontrol unit for determining the sharing ratio of the electric power andcontrolling the electric power distributing device, wherein the controlunit detects a frequency distribution of the output electric power ofthe electric power supply system regarding a magnitude of the electricpower, which includes an assist region and a fuel cell following region,the fuel cell following region having a high frequency of the magnitudeof the electric power, and an assist region having a low frequency ofthe magnitude of the electric power, wherein the control unit determinesto operate the electric power distributing device to output the electricpower from the electric power generator and the electricity storagedevice at the sharing ratio, and wherein a value of the sharing ratio ofthe electricity storage device in the assist region is higher than avalue of the sharing ratio of the electricity storage device in the fuelcell following region.

The electric power supply system can limit the output electric powersupplied by the electricity storage device in the fuel cell followingregion where a frequency of being output is high in the output frequencydistribution in the predetermined time interval. The electric powersupply system can prevent an excess of electricity discharged by theelectricity storage device and efficiently use the electric power of theelectricity storage device. Further, the electric power supply systemcan reduce a frequency of running out of an assisting operation that theelectricity storage device assists generation of electricity byproviding an output electric power for an electrical load, and canimprove a fuel consumption of the electric power generator.

The control unit generates a following upper limit value by adding anaverage value of the electric power of the electric power supply systemand a value corresponding to a standard deviation determined by thefrequency distribution for the electric power, wherein, when theelectric power of the electric power supply system is lower than thefollowing upper limit value, a high frequency of a magnitude of theelectric power is in the fuel cell following region of the frequencydistribution of the electric power of the electric power supply systemregarding the magnitude of the electric power, and wherein, when theelectric power of the electric power supply system is equal to or higherthan the following upper limit value, a low frequency of the magnitudeof the electric power is in the assist region of the frequencydistribution of the output electric power of the electric power supplysystem.

The electric power generator is a fuel cell, and the control unitcomputes an efficiency of the output electric power of the fuel cell inoperation and takes a following lower limit value for a value of anoutput electric power of the fuel cell to maximize the efficiency of theoutput electric power of the fuel cell, and takes the following lowerlimit value for a predetermined value of the output electric power in alight-load region where the fuel cell is deteriorated, and wherein, whenthe output electric power of the fuel cell is equal to or higher thanthe following lower limit value, the control unit determines thedistribution of the output electric power to the fuel cell and theelectricity storage device so as to increase and decrease the outputelectric power supplied by the fuel cell in accordance with the outputelectric power required for the electric power supply system.

The electric power generator is a fuel cell, and the control unitcomputes an efficiency of the output electric power of the fuel cell inoperation and takes a following lower limit value for an output electricpower value of maximizing the efficiency of the output electric power ofthe fuel cell or for a predetermined output electric power value in alight-load region where the fuel cell is deteriorated, and wherein, whenthe output electric power of the fuel cell is equal to or higher thanthe following lower limit value, the control unit determines thedistribution of the output electric power to the fuel cell and theelectricity storage device so as to increase and decrease the outputelectric power supplied by the fuel cell in accordance with the changeof the output electric power value required for the electric powersupply system.

The electric power supply system can efficiently generate electricity bysetting the following lower limit value of the fuel cell in accordancewith the change in the efficiency of the output electric power of thefuel cell in operation.

The electric power supply system includes an auxiliary device electricpower consumption detecting unit for detecting an electric powerconsumption consumed by auxiliary devices of the fuel cell, and whereinthe control unit computes the efficiency of the output electric power ofthe fuel cell on the basis of the electric power consumption of theauxiliary devices and current-voltage characteristics of the fuel cell,whereof a voltage is detected by a voltage detecting unit, and a currentis detected by a current detecting unit.

The electric power supply system can determine the following lower limitvalue on the basis of the current-voltage characteristics of the fuelcell and the electric power consumption of the auxiliary devices, whichare changed in operation, and can improve the efficiency of the outputelectric power.

The control unit includes a temperature detecting unit for detecting atemperature of the fuel cell and computes the efficiency of the outputelectric power of the fuel cell on the basis of the temperature of thefuel cell.

The electric power supply system can use the temperature of the fuelcell to determine the following lower limit value and improve theefficiency of the output electric power of the fuel cell.

The control unit includes a humidity detecting unit for detecting ahumidity of the fuel cell and computes the efficiency of the outputelectric power of the fuel cell on the basis of the humidity of the fuelcell.

The electric power supply system can use the humidity of the fuel cellto determine the following lower limit value and improve the efficiencyof the output electric power of the fuel cell.

The control unit includes a pressure detecting unit for detecting apressure of the fuel cell and computes the efficiency of the outputelectric power of the fuel cell on the basis of the pressure of the fuelcell.

The electric power supply system can use the pressure of the fuel cellto determine the following lower limit value and improve the efficiencyof the output electric power of the fuel cell.

The control unit stops generating electricity of the fuel cell when avalue of the output electric power required for the fuel cell is lowerthan the following lower limit value.

The electric power supply system can stop generating electricity of thefuel cell when the efficiency of the output electric power is low andefficiently use the output electric power.

The control unit changes the following upper limit value on the basis ofthe temperature of the fuel cell which the temperature detecting unitdetects.

The fuel cell system can immediately warm up the electric powergenerator because the fuel cell can generate electricity above thefollowing upper limit value determined by the standard deviation valueby raising the following upper limit value in dropping a temperature ofthe electric power generator.

The control unit changes the following upper limit value on the basis ofa remaining capacity of the electricity storage device which a remainingcapacity detecting unit detects.

The electric power supply system can prevent the remaining capacity ofthe electricity storage device from reducing radically by raising thefollowing upper limit value in accordance with the remaining capacity ofthe electricity storage device, whereby increasing the output electricpower of the electric power generator.

The electric power supply system of the present invention can determinethe distribution of the output electric power so as to increase anddecrease the output electric power supplied by the electric powergenerator in accordance with the change of the output electric powervalue required for the electric power supply system, in the where thefrequency of being output is equal to or higher than a predeterminedvalue in the frequency distribution of the output electric power, andcan determine the distribution of the output electric power so as toincrease the output electric power supplied by electricity storagedevice, in the assist region where the frequency of being output islower than a predetermined value in the frequency distribution, andaccordingly can prevent the excess of discharging from the electricitystorage device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a fuel cell system of a firstembodiment.

FIG. 2 is a block diagram showing a fuel cell system of a secondembodiment.

FIG. 3 is a diagram showing a characteristic curve of an efficiency ofan output electric power of a fuel cell corresponding to the outputelectric power of the fuel cell, and a characteristic curve of afrequency of the output electric power of the fuel cell systemcorresponding to the output electric power of the fuel cell system ofthe second embodiment.

FIG. 4 is a diagram showing an output electric power required for thefuel cell system of the second embodiment in time series.

FIG. 5 is a block diagram showing a fuel cell system of a thirdembodiment.

FIG. 6 is a diagram showing a characteristic curve of an efficiency ofthe output electric power of the fuel cell corresponding to the outputelectric power of the fuel cell, and a characteristic curve of afrequency of the output electric power of the fuel cell systemcorresponding to the output electric power of the fuel cell system ofthe third embodiment.

FIG. 7 is a diagram showing an output electric power of the fuel cellsystem of the third embodiment in time series.

FIG. 8 is a block diagram showing a fuel cell system of the fourthembodiment.

FIG. 9 is a diagram showing a characteristic curve of an efficiency ofan output electric power of the fuel cell corresponding to the outputelectric power required for the fuel cell.

FIG. 10 is a diagram showing a characteristic curve of a temperaturecorresponding to a following lower limit value.

FIG. 11 is a diagram showing a characteristic curve of a pressurecorresponding to the following lower limit value.

FIG. 12 is a diagram showing a characteristic curve of a humiditycorresponding to the following lower limit value.

FIG. 13 is a block diagram showing a control unit of the fourthembodiment.

FIG. 14 is a flowchart showing an operation of the fuel cell system ofthe fourth embodiment.

FIG. 15 is a flowchart showing an operation of an upper/lower limitvalue filter of the control unit of the fourth embodiment.

FIG. 16 is a block diagram showing a fuel cell system of a fifthembodiment.

FIG. 17 is a block diagram showing a control unit of the fifthembodiment.

FIG. 18 is a block diagram showing a fuel cell system of a sixthembodiment.

FIG. 19 is a flowchart showing an operation of a control unit of thesixth embodiment.

FIG. 20 is a block diagram showing a fuel cell system of a seventhembodiment.

FIG. 21 is a diagram showing a characteristic curve of a correctedfollowing upper limit value corresponding to a temperature of the fuelcell.

FIG. 22 is a diagram showing a characteristic curve of the correctedfollowing upper limit value corresponding to a SOC value of a battery.

FIG. 23 is a block diagram showing a control unit of the seventhembodiment.

FIG. 24 is a diagram showing a light-load region and a characteristiccurve of the efficiency of the output electric power of the fuel cell ofthe sixth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will be described with reference to drawings indetail.

First Embodiment

FIG. 1 shows a fuel cell system of a first embodiment. The fuel cellsystem (electric power supply system) of the first embodiment isapplied, for example, to an electric vehicle, a boat or the like, whichoperates with a fuel cell provided as a power supply.

As shown in FIG. 1, the fuel cell system (FC(fuel cell) system:hereinafter referred to as the system) includes: a fuel cell (FC) 101; abattery 102 as a chargeable device (electricity storage device); abattery control unit 103; an electric power distributing device 104; aplurality of auxiliary devices 105; an auxiliary device control unit106; a motor 107 as an electrical load; a motor control unit 108; and acontrol unit 109.

The battery 102 and the fuel cell 101 are connected in parallel, andalso connected to the electric power distributing device 104. Thebattery control unit 103 is connected to the battery 102. The auxiliarydevice control unit 106 is connected to the plurality of auxiliarydevices 105. The motor control unit 108 is connected to the motor 107.The electric power distributing device 104 is connected to the pluralityof auxiliary devices 105 and the motor control unit 108. The electricpower distributing device 104 is connected to the fuel cell 101. Thecontrol unit 109 is connected to the battery control unit 103, theelectric power distributing device 104, the auxiliary device controlunit 106, and the motor control unit 108. As described later, the fuelcell 101 is connected to the control unit 109 via a voltage detector 110and a current detector 111.

The battery 102 is a high-voltage battery providing a storage batterywhich assembles a plurality of unit cells, such as a Li-ion(Lithium-ion) battery or a NiH (nickel hydrogen) battery.

The fuel cell 101 generates electricity by chemical reaction between afuel gas and an oxidation gas, provides an electric power for anelectrical load and connects with the battery 102 in parallel. Thebattery 102 can be charged by the fuel cell 101 and supply an electricpower to electrical loads. The electric power distributing device 104distributes the output electric power of the fuel cell 101 and thebattery 102 on the basis of a control signal from the control unit 109,thereby supplying an electric power to the electrical loads. Theelectrical loads include the plurality of auxiliary devices foroperating to generate electricity. The motor 107 generates a drivingforce to drive a vehicle. The control unit 109 controls the operation ofthe whole fuel cell system.

The fuel cell system further includes the voltage detector 110 and thecurrent detector 111 which are connected to the fuel cell 101. Thevoltage detector 110 and the current detector 111 are connected to thecontrol unit 109. The voltage detector 110 detects a value of the outputvoltage of the fuel cell 101 and transmits the voltage value to thecontrol unit 109. The current detector 111 detects a value of the outputcurrent of the fuel cell 101 and transmits the current value to thecontrol unit 109.

The control unit 109 collects and stores output electric power values ofthe fuel cell system for a predetermined time interval and determines afrequency distribution of the output electric power values. In a fuelcell following region where a frequency of the output electric powervalue in the frequency distribution is equal to or higher than apredetermined value, the control unit 109 determines a sharing ratio ofthe output electric power so as to increase and decrease the outputelectric power supplied by the fuel cell 101 in accordance with theoutput electric power required for the fuel cell system. The controlunit 109 changes the output electric power of the fuel cell 101 inaccordance with the output electric power required for the fuel cellsystem. Further, in an assist region where a frequency of the outputelectric power in the frequency distribution in the predetermined timeinterval is lower than a predetermined value, the control unit 109determines the distribution of the output electric power to the fuelcell 101 and the battery 102 so as to increase the distribution of theoutput electric power supplied by the battery 102.

The output electric power of the fuel cell system (hereinafter referredto as an FC system output or a system output) is equal to the sum of theoutput electric power consumed by the motor 107 and the auxiliarydevices.

The motor 107 in FIG. 1 is used to drive wheels of the vehicle or ascrew propeller of a boat. The auxiliary device 105 is exemplified as anair pump which feeds air to the fuel cell 101 to be driven.

The control unit 109 determines the distribution of output electricpower to the fuel cell 101 and the battery 102 which supply the outputelectric power to the motor 107 via the electric power distributingdevice 104.

The fuel cell system of the first embodiment can suppress the outputelectric power supplied by an electricity storage battery in the fuelcell following region where the frequency of the output electric powerin a predetermined time interval is higher than a predetermined value.Consequently, the fuel cell system can prevent the excess of dischargingfrom the electricity storage battery and efficiently use the outputelectric power of the electricity storage battery. Therefore, the fuelcell system can decrease the frequency of running out of an assistingoperation that the electricity storage battery assists generation ofelectricity by providing the output electric power for an electricalload, and can improve a fuel consumption of the fuel cell.

Second Embodiment

Next, a second embodiment will be described with reference to drawingsin detail. FIG. 2 shows a fuel cell system of the second embodiment. Thesame parts are designated as the same references of the first embodimentand duplicated descriptions thereof will be omitted.

As shown in FIG. 2, the fuel cell system of the second embodimentincludes a control unit 201.

The control unit 201 includes an input unit 202, a computing unit 203and an electric power distributing deciding unit 204. The input unitincludes a motor electric power consumption detector 2021, an auxiliarydevice electric power consumption detector 2022, and an FC (fuel cell)information detector 2023. The FC information detector 2023 receivessignals of a voltage value and a current value of the fuel cell 101.

The computing unit 203 includes a system demand output computing unit2031, an average value computing unit 2032, a system output frequencycomputing unit 2033, an FC efficiency computing unit 2034, a standarddeviation computing unit 2035, an IV estimating unit 2036, and afollowing upper limit value computing unit 2037. The electric powerdistributing deciding unit 204 determines the distribution of the outputelectric power to the fuel cell 101 and the battery 102, from which theoutput electric power is transmitted to the electric power distributingdevice 104, on the basis of a result computed by the computing unit 203.

The system demand output computing unit 2031 computes the outputelectric power required for the fuel cell system. The average valuecomputing unit 2032 computes an average value of the output electricpower of the fuel cell system during a predetermined time interval. Thesystem output frequency computing unit 2033 computes the frequencydistribution of the output electric power of the FC system during thepredetermined time interval. The FC efficiency computing unit 2034computes an efficiency of the output electric power (fuel cell output)of the fuel cell 101. The efficiency of the fuel cell output isdetermined by an equation (F1) as follows;

Efficiency Of Fuel Cell Output=(Output Electric Power Of FuelCell−Output Electric Power Of Auxiliary Device)÷Hydrogen CombustionEnergy  (F1)

JP2006-196221A discloses how to calculate the hydrogen combustion energyand detailed descriptions will be omitted. The output electric power ofthe fuel cell is determined by multiplying a voltage value of the fuelcell by a current value of the fuel cell.

The standard deviation computing unit 2035 computes a standard deviationfrom an average value of the frequency distribution of the outputelectric power of the FC system. The IV estimating unit 2036 estimatescurrent-voltage characteristics (IV characteristics: characteristics ofthe generation of electricity) on the basis of the voltage value and thecurrent value of the fuel cell 101. JP2006-196221A discloses how toestimate the current-voltage characteristics and detailed descriptionsthereof will be omitted. The following upper limit value computing unit2037 computes an upper limit value which is changed in accordance withthe output electric power required for the fuel cell system, the upperlimit value (hereinafter referred to as a following upper limit value)adding the average value determined by the average value computing unit2032 and a standard deviation value determined by the standard deviationcomputing unit 2035.

The average value of the output electric power of the FC system may beapproximately determined by using a first order lag filter, which canreduce the capacity of a memory. The standard deviation may beapproximately determined by using the first order lag filter as well.

Next, the operation of the fuel cell system of the second embodimentwill be described with reference to FIG. 2, FIG. 3, and FIG. 4.

FIG. 3 shows the operation of the fuel cell system. In FIG. 3, avertical axis shows the efficiency of the output electric power of thefuel cell and the frequency of the output electric power of the fuelcell system, and a horizontal axis shows the output electric power ofthe fuel cell (FC output) and the output electric power of the fuel cellsystem (system output). A characteristic curve A1 shows the efficiencyof the output electric power of the fuel cell, which is corresponding tothe FC output. A characteristic curve B1 shows a histogram of theelectric power consumption of the fuel cell system, which iscorresponding to the system output.

FIG. 4 shows the output electric power of the fuel cell system in timeseries. In FIG. 4, a vertical axis shows the output electric powerrequired for the fuel cell system, and a horizontal axis shows time.

As shown in FIG. 3 and FIG. 4, the control unit 201 computes an averagevalue and a standard deviation value of the output electric power of thefuel cell system in a predetermined time interval on the basis of thefrequency distribution of the output electric power of the fuel cellsystem, and computes the following upper limit value by adding theaverage value and the standard deviation value. When the output electricpower required for the fuel cell system is lower than the followingupper limit value (fuel cell following region), the control unit 201determines the distribution of the output electric power to the fuelcell 101 and the battery 102 so as to increase and decrease the outputelectric power supplied by the fuel cell 101 in accordance with a changeof the output electric power required for the fuel cell system. When theoutput electric power required for the fuel cell system is equal to orhigher than the following upper limit value (assist region), the controlunit 201 determines the sharing ratio of the output electric power tothe fuel cell 101 and the battery 102 so as to increase the distributionof the output electric power supplied by the battery 102. In the assistregion, the control unit 201 changes the sharing ratio of the outputelectric power to the fuel cell 101 and the battery 102 in accordancewith the output electric power required for the fuel cell system.Further, the control unit 201 transmits the distribution of the outputelectric power to the electric power distributing device 104 beingcontrolled.

It is not necessary that the control unit 201 distributes the outputelectric power so as to equate the output electric power of the fuelcell 101 to the output electric power required for the fuel cell system.The output voltage of the fuel cell can be increased by adding ahigh-voltage gain.

The electric power distributing device 104 controlled by the controlunit 201 of the second embodiment distributes the output electric powerof the fuel cell 101 as shown in a solid line C1 in FIG. 4 and theoutput electric power of the battery 102 as shown in a chain line D1 inFIG. 4.

The fuel cell system of the second embodiment can compute the averagevalue and the standard deviation value on the basis of the frequencydistribution of the output electric power of the fuel cell system, andcompute the following upper limit value by adding the average value andthe standard deviation value, and determine the following upper limitvalue in accordance with the change of the output electric power of thefuel cell system in time series. Accordingly, the fuel cell system canspecify a region where the output electric power of the fuel cell systemis frequently used, although the average value and the standarddeviation value on the basis of the frequency distribution of the outputelectric power of the fuel cell system in a predetermined time intervalare changed as time goes by.

Next, a third embodiment will be described with reference to drawings indetail. FIG. 5 shows the fuel cell system of the third embodiment. Thesame parts are designated as the same references of the first and secondembodiments and duplicated descriptions thereof will be omitted.

As shown FIG. 5, the fuel cell system of the third embodiment includes acontrol unit 301 which includes a following lower limit value computingunit 3011.

The following lower limit value computing unit 3011 computes an averagevalue and a standard deviation value on the basis of the frequencydistribution of the output electric power of the fuel cell system in apredetermined time interval, and computes a following lower limit valueby subtracting the standard deviation value from the average value.

Next, the operation of the fuel cell system of the third embodiment willbe described with reference to FIG. 5, FIG. 6 and FIG. 7.

FIG. 6 shows the operation of the fuel cell system. In FIG. 6, avertical axis shows an efficiency of the output electric power of thefuel cell and a frequency of the output electric power of the fuel cellsystem, and a horizontal axis shows the output electric power of thefuel cell (FC output) and the output electric power of the fuel cellsystem (system output). Characteristic curves A2 and B2 aresubstantially the same as those of FIG. 3 and descriptions thereof willbe omitted.

As shown in FIG. 6, when the output electric power required for the fuelcell system is equal to or higher than the following lower limit value,and is lower than the following upper limit value, the control unit 301determines the distribution of the output electric power to the fuelcell 101 and the battery 102 so as to increase and decrease the outputelectric power supplied by the fuel cell 101 in accordance with a changeof the output electric power required for the fuel cell system. When theoutput electric power required for the fuel cell system is equal to orhigher than the following upper limit value, and is lower than thefollowing lower limit value (assist region), the control unit 301determines the distribution of the output electric power to the fuelcell 101 and the battery 102 so as to increase the distribution of theoutput electric power supplied by the battery 102. Further, the controlunit 301 transmits the signal of a distribution value of the outputelectric power to the electric power distributing device 104 beingcontrolled.

FIG. 7 shows the output electric power values of the fuel cell system intime series. In FIG. 7, a vertical axis shows the output electric powerrequired for the fuel cell system, and a horizontal axis shows time.

The electric power distributing device 104 controlled by the controlunit 301 of the third embodiment distributes the output electric powerof the fuel cell 101 as shown in a solid line E1 in FIG. 7 and theoutput electric power of the battery 102 as shown in a chain line F1 inFIG. 7. The control unit 301 controls the output electric power of theFC system in such a manner that the output electric power of the FCsystem is higher than the following lower limit value (lower assistregion in FIG. 7). When the output electric power of the FC system isequal to or lower than the following lower limit value, the control unit301 can instruct the electric power distributing device 104 to chargethe battery 102 with a surplus output electric power of the fuel cell(as shown in a dashed line F2 in FIG. 7).

The fuel cell system of the third embodiment can compute the averagevalue and the standard deviation value on the basis of the frequencydistribution of the output electric power of the fuel cell system, andcompute the following lower limit value by subtracting the standarddeviation value from the average value, and determine the followinglower limit value in accordance with a change of the output electricpower of the fuel cell system in time series. Accordingly, the fuel cellsystem can specify a region where the output electric power of the fuelcell system is frequently used, although the average value and thestandard deviation value on the basis of the frequency distribution ofthe output electric power of the fuel cell system in a predeterminedtime interval are changed as time goes by.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be describedwith reference to drawings in detail. FIG. 8 shows a fuel cell system ofthe fourth embodiment. The same parts are designated as the samereferences of the first embodiment and duplicated descriptions thereofwill be omitted.

As shown in FIG. 8, the fuel cell system of the fourth embodimentincludes an auxiliary device consuming electric power detector 601, atemperature detector 602, a pressure detector 603, a humidity detector604, a control unit 605, and the same parts except the control unit 109of the first embodiment.

The auxiliary device consuming electric power detector 601 connects tothe auxiliary device control unit 106, detects an electric powerconsumed by the auxiliary devices 105 (FIG. 1) of the fuel cell system,and transmits the signal of the electric power value to the control unit605. The temperature detector 602 detects an internal temperature valueof the fuel cell 101 and transmits the signal of the internaltemperature value to the control unit 605. The pressure detector 603detects an internal pressure value of the fuel cell 101 and transmitsthe signal of the internal pressure value to the control unit 605. Thehumidity detector 604 detects an internal humidity value of the fuelcell 101 and transmits the signal of the internal humidity value to thecontrol unit 605.

The control unit determines the following upper limit value as describedin the first and second embodiments, computes an efficiency of theoutput electric power of the fuel cell when the fuel cell systemperiodically supplies an output electric power to the motor 107, anddetermines the following lower limit value which is a value of theoutput electric power of the fuel cell 101 to maximize the efficiency ofthe output electric power of the fuel cell. When the output electricpower of the fuel cell 101 is equal to or higher than the followinglower limit value, and the output electric power required for the fuelcell system is lower than the following upper limit value, the controlunit 605 determines the distribution of the output electric power to thefuel cell 101 and the battery 102 so as to increase and decrease theoutput electric power supplied by the fuel cell 101 in accordance with achange of the output electric power required for the fuel cell system.

In this case, the control unit 605 computes the efficiency of the outputelectric power of the fuel cell on the basis of current-voltagecharacteristics of the fuel cell and the electric power consumption ofthe auxiliary devices.

FIG. 9 shows an operation of the fuel cell of the fourth embodiment. InFIG. 9, a vertical axis shows the efficiency of the output electricpower of the fuel cell, and a horizontal axis shows the output electricpower of the fuel cell (FC output). A characteristic curve G1 shows achange in the efficiency of the output electric power of the fuel cell.

The control unit 605 determines the following upper limit value asdescribed in the first and second embodiments, determines the followinglower limit value which is a value of the output electric power of thefuel cell 101 to maximize the efficiency of the output electric power ofthe fuel cell and shown as an FC output value when the efficiency of theFC output is the highest on the characteristic curve G1 of FIG. 9. Whenthe output electric power of the fuel cell 101 is equal to or higherthan the following lower limit value, the output electric power requiredfor the fuel cell system is lower than the following upper limit value,the control unit 605 determines the distribution of the output electricpower to the fuel cell 101 and the battery 102 so as to increase anddecrease the output electric power supplied by the fuel cell 101 inaccordance with a change of the output electric power required for thefuel cell system.

The control unit 605 may as well compute the efficiency of the outputelectric power of the fuel cell on the basis of any of a temperature ofthe temperature detector 602, a pressure of the pressure detector 603,or a humidity of the humidity detector 604. It is because the efficiencyof the output electric power of the fuel cell is changed with any of thetemperature of the temperature detector 602, the pressure of thepressure detector 603, or the humidity of the humidity detector 604, andaccordingly, the following lower limit value is changed.

As shown in FIG. 10 having a characteristic curve H1, the followinglower limit value is changed on the basis of the temperature of thetemperature detector 602. As shown in FIG. 11 having a characteristiccurve I1, the following lower limit value is changed on the basis of thepressure of the pressure detector 603. As shown in FIG. 12 having acharacteristic curve J1, the following lower limit value is changed onthe basis of the humidity of the humidity detector 604.

The control unit 605 may as well instruct the fuel cell 101 to stopgeneration of electricity when the output electric power required forthe fuel cell 101 is lower than the following lower limit value.

Next, the detail of the control unit 605 of the fourth embodiment willbe described with reference to FIG. 13. FIG. 13 shows the control unit605 of the fourth embodiment of the fuel cell system.

The control unit 605 includes; a system demand output computing unit401; an average value computing unit 402; a standard deviation computingunit 403; adders 404 and 410; an upper/lower limit value filter 405;subtractors 406 and 408; an assist protecting control unit 407; anassist ability lacking value deciding unit 409; an IV estimating unit411; a lower limit value output graph searching unit 413; an assistcalculating value computing unit 414; and an electric power distributiondeciding unit 204. Functions of the units 401 to 414 will be describedlater with reference to FIG. 14.

Next, an operation of the control unit 605 of the fourth embodiment ofthe fuel cell system will be described with reference to FIG. 13 andFIG. 14 in detail. FIG. 14 is a flowchart showing the operation of thecontrol unit 605 of the fourth embodiment of the fuel cell system.

As shown in FIG. 14, in a step ST1, the control unit 605 determineswhether or not an ignition is turned on. When the ignition is not turnedon, processing returns to the step ST1 (step 1, No).

In the step ST1, when the ignition is turned on (step ST2, Yes), thesystem demand output computing unit 401 receives a signal of an electricpower value of the auxiliary device estimated by the auxiliary devicecontrol unit 106 and a signal of an electric power value of the motor107 estimated by the motor control unit 108, and computes an outputelectric power value required for the fuel cell system. The systemdemand output computing unit 401 stores the output electric power valuerequired for the fuel cell system in a memory not shown. The systemdemand output computing unit 401 computes an output electric power valueof the fuel cell 101 (FC output) on the basis of values of the voltagedetector 110, the current detector 111 and the auxiliary deviceconsuming electric power detector 601, and transmits the signal of theoutput electric power value of the fuel cell 101 to the upper/lowerlimit value filter 405. After a predetermined time interval, the averagevalue computing unit 402 and the standard deviation computing unit 403receive the signal of the output electric power value required for thefuel cell system of a predetermined time period from the memory notshown. Further, the average value computing unit 402 receives the signalof a present output electric power value required for the fuel cellsystem from the system demand output computing unit 401, uses the outputelectric power value required for the fuel cell system of thepredetermined time period from the memory not shown, and computes anaverage value of the output electric power required for the fuel cellsystem (demanded system output power) of the predetermined time period(step ST2). Upon computing the average value, the average valuecomputing unit 402 stores the present output electric power valuerequired for the fuel cell system in the memory not shown.

Next, in a step ST3, upon receiving the signal of the present outputelectric power value required for the fuel cell system from the systemdemand output computing unit 401, the standard deviation computing unit403 uses the output electric power value required for the fuel cellsystem of the predetermined time period from the memory not shown, andcomputes a standard deviation value of the output electric powerrequired for the fuel cell system (demanded system output power) of thepredetermined time period. Subsequently, in a step ST4, the adder 404(corresponding to the following upper limit value computing unit 2037 inFIG. 2) computes a following upper limit value which adds the averagevalue computed by the average value computing unit 402 and the standarddeviation value computed by the standard deviation computing unit 403.

In a step ST5, an FC efficiency computing unit 412 computes anefficiency of the output electric power of the fuel cell 101 on thebasis of a current value of the current detector 111 and a voltage valueof the voltage detector 110. In a step ST6, the lower limit value outputgraph searching unit 413 searches following lower limit values inaccordance with the efficiency of the output electric power of the fuelcell 101, computed by the FC efficiency computing unit 412, in graphs ofFIG. 9 to FIG. 12.

In a step ST7, the upper/lower limit value filter 405 receives thesignal of the output electric power value required for the fuel cellsystem, and transmitted by the system demand output computing unit 401as a target value for generating electricity. The upper/lower limitvalue filter 405 filters the output electric power value required forthe fuel cell system, which is the target value, by cutting off abovethe following upper limit value of the adder 404 and below the followinglower limit value of the lower limit value output graph searching unit413, and computes a temporary output electric power value of the fuelcell system. The step ST7 will be described later with reference to FIG.15.

In a step ST8, the subtractor 406 computes a temporary assistcalculating value by subtracting the temporary output electric powervalue of the fuel cell system computed by the upper/lower limit valuefilter 405 from the output electric power value required for the fuelcell system, transmitted by the system demand output computing unit 401.In a step ST9, the subtractor 408 computes a temporary assist abilitylacking value by subtracting an assist limit value of the assistprotecting control unit 407 from the temporary assist calculating valueof the subtractor 406.

In a step ST10, upon receiving the signal of the temporary assistability lacking value of the subtractor 408, the assist ability lackingvalue deciding unit 409 determines whether or not the temporary assistability lacking value is larger than zero. When the temporary assistability lacking value is larger than zero (step ST10, Yes), the assistability lacking value deciding unit 409 takes the temporary assistability lacking value for an assist ability lacking value (step ST11).Processing proceeds to a step ST13. When the temporary assist abilitylacking value is lower than zero (step ST10, No), the assist abilitylacking value deciding unit 409 takes the temporary assist abilitylacking value for zero (step ST12). Processing proceeds to the stepST13.

In the step ST13, the adder 410 compute an estimated output electricpower value required for the fuel cell system by adding the temporaryoutput electric power value of the fuel cell system of the upper/lowerlimit value filter 405 and the assist ability lacking value of theassist ability lacking value deciding unit 409, and transmits the signalof the estimated output electric power value to the assist calculatingvalue computing unit 414. In a step ST14, the assist calculating valuecomputing unit 414 computes an assist calculating value by subtractingthe estimated output electric power value of the adder 410 from theoutput electric power value required for the fuel cell system, which istransmitted by the system demand output computing unit 401, andtransmits the signal of the assist calculating value to the electricpower distributing deciding unit 204. Subsequently, the electric powerdistributing deciding unit 204 determines the distribution of the outputelectric power supplied to the motor 107 to the fuel cell 101 and thebattery 102 on the basis of the estimated output electric power valuerequired for the fuel cell system and the assist calculating value.

In a step ST15, the control unit 301 determines whether or not theignition is turned off. When the ignition is turned off (step ST15,Yes), the control unit 301 stops the operation. When the ignition is notturned off (step ST15, No), processing returns to the step ST2.

In the flowchart of FIG. 14, when the following lower limit value iscomputed by subtracting the standard deviation value from the averagevalue of the output electric power required for the fuel cell system,and the estimated output electric power required for the fuel cellsystem is computed by cutting off below the following lower limit value,the step proceeds as the same steps where the control unit 301 controlsin the third embodiment. The following lower limit value computed basedon the average value and the standard deviation value may as well becorrected in searching the graphs in the step ST16.

Next, an operation of the upper/lower limit value filter 405 of thefourth embodiment will be described with reference to FIG. 13 and FIG.15 in detail. FIG. 15 shows a flowchart of the operation of theupper/lower limit value filter 405.

First, the upper/lower limit value filter 405 receives the outputelectric power value required for the fuel cell system (step ST101).

Subsequently, the upper/lower limit value filter 405 compares the outputelectric power value required for the fuel cell system (input value),which is transmitted by the system demand output computing unit 401,with the following upper limit value, and determines whether or not theoutput electric power value required for the fuel cell system (inputvalue) is lower than the following upper limit value (step ST102).

In the step ST102, when the output electric power value required for thefuel cell system (input value) is equal to or higher than the followingupper limit value (step ST102, No), the upper/lower limit value filter405 outputs the following upper limit value (step ST103).

In the step ST102, when the output electric power value required for thefuel cell system (input value) is lower than the following upper limitvalue (step ST102, Yes), the upper/lower limit value filter 405 comparesthe output electric power value required for the fuel cell system (inputvalue), which is transmitted by the system demand output computing unit401, with the following lower limit value, and determines whether or notthe output electric power value required for the fuel cell system (inputvalue) is larger than the following lower limit value (step ST104).

In the step ST104, when the output electric power value required for thefuel cell system (input value) is lower than the following lower limitvalue (step ST104, No), the upper/lower limit value filter 405 outputsthe following lower limit value (step ST105).

In the step ST104, when the output electric power value required for thefuel cell system (input value) is larger than the following upper limitvalue (step ST104, Yes), the upper/lower limit value filter 405 outputsthe output electric power value required for the fuel cell system (inputvalue) (step ST106).

The upper/lower limit value filter 405 transmits the signals of theoutput values of step ST103, ST105, and ST106 as a temporary outputelectric power value of the fuel cell system (step ST107).

The fuel cell system of the fourth embodiment can efficiently generatethe electricity of the fuel cell 101 by setting a lower limit value ofgenerating the electricity of the fuel cell in accordance with a changeof the efficiency of the output electric power of the fuel cell inoperation.

The fuel cell system can compute the efficiency of the output electricpower of the fuel cell in accordance with a charging state of the fuelcell 101 on the basis of the current-voltage characteristics of the fuelcell 101, the electric power consumption of auxiliary devices, thetemperature, the pressure, or the humidity of the fuel cell 101.

Further, the fuel cell system can efficiently use the energy of the fuelcell 101 because the system stops generating the electricity of the fuelcell when the output electric power of the fuel cell 101 is lower thanthe following lower limit value.

Fifth Embodiment

Next, a fifth embodiment will be described with a reference FIG. 16 andFIG. 17 in detail. FIG. 16 shows a fuel cell system of the fifthembodiment. FIG. 17 shows a control unit of the fuel cell system of thefifth embodiment. The same parts are designated as the same referencesof the third embodiment and duplicated descriptions thereof will beomitted.

As shown in FIG. 16, the fuel cell system of the fifth embodimentincludes a control unit 501.

A gain map 503 computes a predetermined gain value on the basis of anaverage value of the average value computing unit 402 and an SOC value(state of charge) of the battery control 103, and transmits thepredetermined gain value to an adder 504. The adder 504 adds thepredetermined gain value to the output electric value required for thefuel cell system, transmitted by the system demand output computing unit401, generates a target value of generating electricity of the fuel cell101, and transmits the signal of the target value to the upper/lowerlimit value filter 405 and the assist calculating value computing unit414. A gain buffer 505 adds a predetermined gain value to the followingupper limit value of the adder 404, and transmits the signal of theadded value to the upper/lower limit value filter 405.

An SOC correcting unit 502 receives the signals of the SOC value of thebattery control 103 and the following lower limit value of theupper/lower limit value filter 405, and corrects the following lowerlimit value on the basis of the SOC value, and transmits the signal ofthe corrected following lower limit value to the lower limit valueoutput graph searching unit 413. When the SOC value is high, the SOCcorrecting unit 502 makes a correction value low. When the SOC value islow, the SOC correcting unit 502 makes the correction value high.

In FIG. 17, the control unit 501 may as well delete the gain map 503,the adder 504, and the gain buffer 505.

The fuel cell system of the fifth embodiment can set the following lowerlimit value in accordance with a change of the SOC value, and achievethe high efficiency of the output electric power of the fuel cellsystem.

The fuel cell system can generate an adjusted following upper limitvalue in accordance with the characteristics of the fuel cell system byadding the predetermined gain value to the following upper limit value.

Sixth Embodiment

Next, a sixth embodiment will be described with reference to FIG. 18.FIG. 18 shows a fuel cell system of the sixth embodiment. The same partsare designated as the same references of the first to fifth embodimentsand duplicated descriptions thereof will be omitted.

As shown in FIG. 18, the fuel cell system of the sixth embodimentincludes a control unit 701, a past record referring unit 205, and apast record memorizing unit 510, along with same parts of the thirdembodiment.

The past record memorizing unit 510 stores a previous driving averagevalue 511, a previous driving standard deviation value 512, an averagingdriving average value 513, and an average driving standard deviationvalue 514. The average value of the sixth embodiment is same as theaverage value of the output electric power of the fuel cell system inthe predetermined time interval in the second embodiment. The standarddeviation value is same as the standard deviation value from the averagevalue of the fuel cell system of the second embodiment. The previousdriving average value 511 is an average value of the output electricpower required for the fuel cell system, which is computed by theaverage value computing unit 2032 during a previous driving. Theprevious driving standard deviation value 512 is the standard deviationvalue of the output electric power of the fuel cell system, which iscomputed by the standard deviation computing unit 2035 during theprevious driving. The previous driving average value 511 is renewedevery time the average value computing unit 2032 computes the averagevalue of the output electric power of the fuel cell system. The previousdriving standard deviation value 512 is renewed every time the standarddeviation computing unit 2035 computes the standard deviation value ofthe output electric power of the fuel cell system.

The averaging driving average value 513 and the average driving standarddeviation value 514 are computed in a laboratory in advance and storedin the past record memorizing unit 510 via the input unit.

In the sixth embodiment, the previous driving average value 511 and theprevious driving standard deviation value 512 are described as “learnedvalues” for the convenience sake.

When the ignition is turned on, the past record referring unit 205 getsnecessary data stored in the past record memorizing unit 510 andinitializes the average value and the standard deviation value of theoutput electric power of the fuel cell system.

Next, an operation of the control unit 701 of the fuel cell system ofthe sixth embodiment will be described with reference to FIG. 18 andFIG. 19 in detail. FIG. 19 shows a flowchart of the operation of thecontrol unit 701. Steps of FIG. 19 follow the step ST1 of FIG. 14.

As shown in FIG. 19, in a step ST201, the control unit 701 determineswhether or not the ignition has just been turned on. When the ignitionhas not just been turned on (step ST1, No), processing proceeds to thestep ST2 of FIG. 14 (step ST202).

In the step ST201, when the ignition has just been turned on (stepST201, Yes), the past record referring unit 205 determines whether ornot the learned values (the previous driving average value 511 and theprevious driving standard deviation value 512) are “NULL” (step ST203).For example, when the learned values stored in a memory are deleted byplugging off a battery, the learned values take on “NULL”.

When the learned values are not “NULL” (step ST203, No), the past recordreferring unit 205 substitutes the previous driving standard deviationvalue 512 of the past record memorizing unit 510 for an initial standarddeviation value of the output electric power required for the fuel cellsystem (step ST205). Processing proceeds to the step ST2 of FIG. 14(step ST202).

When the learned values are “NULL” (step ST203, Yes), the past recordreferring unit 205 substitutes the averaging driving average value 513of the past record memorizing unit 510 for an initial average value ofthe output electric power required for the fuel cell system (stepST206). The past record referring unit 205 substitutes the averagedriving standard deviation value 514 of the past record memorizing unit510 for an initial standard deviation value of the output electric powerrequired for the fuel cell system (step ST206). Processing proceeds tothe step ST2 of FIG. 14 (step ST202).

The fuel cell system of the sixth embodiment can store the average valueand standard deviation value of the output electric power required forthe fuel cell system during the previous driving, as learned values, andtake the average value and standard deviation value for initial valuesfor the next driving. Accordingly, when a similar driving pattern isrepeated, the fuel cell system can efficiently generate electricity at astarting time of driving.

The past record memorizing unit 510 may as well store the followingupper limit value and the following lower limit value computed duringthe previous driving as the learned values.

Seventh Embodiment

A seventh embodiment will be described with reference to FIG. 20. FIG.20 is a block diagram showing a fuel cell system of the seventhembodiment. The same parts are designated as the same references of thefirst to sixth embodiments and duplicated descriptions thereof will beomitted.

As shown in FIG. 20, the fuel cell system of the seventh embodimentincludes a control unit 801.

The fuel cell system includes the voltage detector 110, the currentdetector 109 and an FC temperature detector 112. The control unit 801 isconnected to the voltage detector 110, the current detector 109 and theFC temperature detector 112. The FC temperature detector 112 detects atemperature of the fuel cell 101 and transmits a signal of thetemperature value to the control unit 801.

The control unit 801 includes an input unit 202, which includes a motorelectric power consumption detector 2021, an auxiliary device electricpower consumption detector 2022, an FC (fuel cell) information detector2023′ and a battery information detector 2024 which receives a signal ofSOC information of the battery 102 from a battery control unit 103. TheFC information detector 2023′ receives the signals of a voltage valueand a current value of the fuel cell 101 and the signal of a temperatureof the fuel cell detected by the FC temperature detector 112. Afollowing upper limit value computing unit 2037′ includes anSOC/temperature correcting graph searching unit 415 and a multiplyingunit 416, which are described later in relation to FIG. 23.

FIG. 21 and FIG. 22 are graphs showing an operation of the fuel cellsystem of the seventh embodiment, which illustrates a correctedfollowing upper limit value. In FIG. 21, a vertical axis shows acorrected following upper limit value, and a horizontal axis shows atemperature (FC temperature) of the fuel cell 101 (FIG. 20). As shownwith a characteristic curve K1 in FIG. 21, the corrected following upperlimit value increases as the temperature of the fuel cell drops.

In FIG. 22, a vertical axis shows the corrected following upper limitvalue, and a horizontal axis shows an SOC (state of charge) value of thebattery 102 (FIG. 20). As shown with a characteristic curve L1 in FIG.22, the corrected following upper limit value increases as the SOC valueof the battery 102 drops.

FIG. 23 is a block diagram showing the control unit 801 of the fuel cellsystem of the seventh embodiment.

As shown in FIG. 23, the control unit 801 includes a SOC/temperaturecorrecting graph searching unit 415 (corresponding to the followingupper limit value computing unit 2037′ in FIG. 20), and a multiplyingunit 416 (corresponding to the following upper limit value computingunit 2037′ in FIG. 20), and in addition to the components shown in FIG.13.

The average value computing unit 402 computes an average value collectedby the output electric power required for the fuel cell system for apredetermined time period, and the standard deviation computing unit 403computes a standard deviation value of the output electric powerrequired for the fuel cell system for the predetermined time period. TheSOC/temperature correcting graph searching unit 415 computes thecorrected following upper limit value by searching the graphs of FIG. 21and FIG. 22 on the basis of the SOC value of the battery informationdetector 2024 and the temperature of the fuel cell 101 which the FCinformation detector 2023′ detects. The multiplying unit 416 multipliesthe standard deviation value of the standard deviation computing unit403 by the corrected following upper limit value of the SOC/temperaturecorrecting graph searching unit 415. The adder 404 adds the averagevalue of the average value computing unit 402 to a multiplied value ofthe multiplying unit 416. Subsequently, the adder 404 outputs afollowing upper limit value, which is transmitted to the upper/lowerlimit value filter 405. The following upper limit value is increased, asthe temperature of the fuel cell drops, or as the SOC value of thebattery 102 drops. The subsequent process is same as in FIG. 13 and FIG.14, and thus duplicated descriptions thereof will be omitted.

The fuel cell system of the seventh embodiment can quickly warm up thefuel cell 101 because the fuel cell 101 can generate electricity abovethe following upper limit value determined on the basis of the standarddeviation value by raising the following upper limit value as atemperature of the electric power generator becomes low as shown in FIG.21.

For example, under a low temperature condition, when the output electricpower of the fuel cell 101 is increased so as to be warmed up, a largeamount of surplus electric power (surplus power) is generated. Thesurplus power is stored in the battery 102 (FIG. 20). Accordingly, aremaining capacity of the battery 102 is drastically reduced, the fuelcell cannot be warmed up, and the battery cannot be charged with aregenerated electric power from the motor 107. On the other hand, evenwhen the remaining capacity of the battery is still available, theelectric power generated by the fuel cell 101 is supplied to the motor107 after stored once the battery 102. Consequently, the efficiency ofthe fuel cell system is dropped.

According to the seventh embodiment, the fuel cell following region ofthe fuel cell 101 is expanded upwardly by raising the following upperlimit value as the temperature of the fuel cell lowers to increase theoutput generated electric power of the fuel cell 101. Consequently, thefuel cell system of the seventh embodiment can warm up the fuel cellwhile generating electricity with the fuel cell 101 which is suited fortraveling. Furthermore, as compared with the case where the output powerof the fuel cell 101 itself is increased, the decrease in a remainingcapacity of the battery 102 due to the surplus power is suppressed.Accordingly, there is prevented a case where the battery 102 is fullycharged, may otherwise hamper warming-up of the fuel cell 101 andcollection of the regenerated electric power. In addition the efficiencyof the output electric power is expected to improve.

As shown in FIG. 22, the fuel cell following region of the fuel cell 101is expanded upwardly by raising the following upper limit value as theSOC value of the battery 102 drops. Consequently, the output electricpower of the fuel cell 101 is increased. The fuel cell system canprevent the remaining capacity of the battery 102 from drasticallyreducing due to the surplus power by supplying a part of the increasedoutput electric power to the battery 102.

The various components, such as the electric power distribution decidingunit 204, the system demand output computing unit 2031, the averagevalue computing unit 2032, the system output frequency computing unit2033, the FC efficiency computing unit 2034, the standard deviationcomputing unit 2035, the IV estimating unit 2036, the following upperlimit value computing units 2037 and 2037′, the following lower limitvalue computing unit 3011, the system demand output computing unit 401,the average value computing unit 402, the standard deviation computingunit 403, the adders 404 and 410, the upper/lower limit value filter405, the subtractors 406 and 408, the assist protecting control unit407, the assist ability lacking value deciding unit 409, the IVestimating unit 411, the lower limit value output graph searching unit413, the assist calculating value computing unit 414, the SOC correctingunit 502, the gain map 503, the adder 504, the gain buffer 505 shown inFIG. 2, 5, 13, 17, 20, 23, are realized by implementing programs by aCPU (central processing unit), which are stored in a memory unit, suchas a ROM (read-only memory, not shown) and expanded in a RAM (randomaccess memory).

Modified Embodiment

The fuel cell system of the present invention can prevent an excess ofelectricity discharged by a chargeable device and can be applied to amovable power generator such as a vehicle, a boat, an airplane and aportable power generator or to a household small-size power generator.

In the embodiments of the present invention, the fuel cell systemgenerates the following upper limit value by adding the standarddeviation value to an average value of the output electric power of thefuel cell system in a past predetermined time period and the followlower limit value by subtracting the standard deviation value from theaverage value of the output electric power of the fuel cell system inthe past predetermined time period. However, the present invention isnot limited to the embodiments, but may be modified. For example, thefollowing upper limit value or the following lower limit value can becomputed by adding or subtracting various values such as 1.5σ, 1.96σ, or0.7σ, to or from the average value, respectively, where the standarddeviation value is represented as “a”.

In the embodiments above, the following lower limit value is determinedbased on the graphs of FIG. 9 to FIG. 12, but may be determined based ona graph of FIG. 24.

FIG. 24 shows an operation of the fuel cell system of the fourthembodiment. In FIG. 24, the same parts are designated as the samereferences of FIG. 9 and duplicated descriptions thereof will beomitted. As shown in FIG. 24, the following lower limit value may aswell be determined on the basis of a value on the characteristic curveG1 in a light-load region 5000. An appropriate value in the light-loadregion 5000 can be selected for the following lower limit value on thebasis of an experiment in advance. The light-load region 5000 covers aregion below the output electric power value of the fuel cell system,that attains the highest efficiency on the characteristic curve G1 inFIG. 24. The experiment may be performed in advance to determine theoutput electric power value where the fuel cell 101 begins todeteriorate as will be described later, and in an range of values belowthis output electric power value, the following lower limit value may aswell be set. Accordingly, the following lower limit value can be apredetermined output electric power value in the light-load region wherethe deterioration of the fuel cell progresses. The light-load regionwill be described below.

When sufficient hydrogen is supplied to an anode of the fuel cell 101(the output electric power of the fuel cell is normal), a chemicalreaction represented by the following formula (F2) progresses.

H₂→2H⁺+2e ⁻  (F2)

However, when the output electric power of the fuel cell 101 is low, anda flow rate of hydrogen is low, hydrogen supplied to the anode is notsufficient. In this case, the following reactions represented byformulas (F3) and (F4) progress in the fuel cell 101.

C+H₂O→CO₂+4H⁺+4e ⁺  (F3)

Pt→Pt₂ ⁺+2e ⁺  (F4)

In the formula (F3), “C” indicates the carbon in an electrode. In theformula (F4), “Pt” is a catalysis. When hydrogen is not sufficientlysupplied, electrons of the components of the fuel cell 101 are supplied.Consequently, the fuel cell is deteriorated. The light-load region meansan output region in which insufficient hydrogen supply leads todeterioration of the fuel cell 101.

The following lower limit value is determined as shown in FIG. 24 in thelight-load region where the hydrogen is insufficiently supplied.Accordingly, the deterioration of the fuel cell 101 can be prevented byincreasing the distribution of the output electric power to be suppliedfrom the battery 102 or by stopping the operation of the fuel cell 101.

1. An electric power supply system comprising: an electric powergenerator for generating an electric power; an electricity storagedevice for being charged with the electric power from the electric powergenerator, and supplying the electric power to an electrical load; anelectric power distributing device for distributing the electric powerto be supplied to the electrical load, from the electric power generatorand the electricity storage device at a sharing ratio therebetween; anda control unit for determining the sharing ratio of the electric powerand controlling the electric power distributing device, wherein when anelectric power region of the electric power supply system capable ofsupplying the electric power to the electrical load is at least dividedinto a low output region, a middle output region, and a high outputregion: the middle output region is a region in which an output of theelectric power supply system is frequently; in the middle output region,the electric power of the electric power generator, in comparison withthe electric power of the electricity storage device, follows theelectric power to be supplied to the electrical load; and in the lowoutput region and in the high output region, the electric power of theelectricity storage device, in comparison with the electric power of theelectric power generator, follows the electric power to be supplied tothe electrical load.
 2. The electric power supply system according toclaim 1, wherein the electric power generator is a fuel cell, and amedian value between an upper limit and a lower limit in the middleoutput region is higher than a highest value in an output efficiency ofthe fuel cell.
 3. The electric power supply system according to claim 2,wherein the middle output region is an output region in which the outputefficiency of the fuel cell is equal to or higher than the highest valuein the output efficiency of the fuel cell.